The invention relates to a polymer composition containing a propylene block copolymer, a method for the preparation of moulded parts that contain the polymer composition and moulded parts that contain the polymer composition.
Such a polymer composition is known from EP-A-714.948.
The polymer composition is used, for example, for injection moulding of large moulded parts, such as for example garden tables, garden chairs, car bumpers and instrument panels. This means that major requirements are to be met in terms of, for example, the flow and the mechanical properties of the composition.
It is important that the large moulded parts can be injection moulded using a mould with the lowest possible number of gates. This ensures that the number of weld lines and the associated surface defects and weak spots in the moulded part is kept to a minimum. To achieve this, it is necessary for the polymer composition to be capable of being injection moulded at high injection rates so as to obtain long flow paths, which are needed for injection moulding in a mould using a minimal number of gates.
However, a major drawback of injection moulding at high injection rates is that it results in moulded parts having surface defects in the form of a stripe or wave pattern, which often extends perpendicularly to the direction of flow of the polymer composition in the mould. In practice this effect is also referred to as slipstick.
The aim of the invention is to provide a polymer composition that can be injection moulded at a high injection rate, while the moulded part thus obtained does not exhibit the surface defects, or at least exhibits them to a strongly reduced extent.
This aim is achieved by providing a polymer composition, which contains:
A. 100-40 wt. % of a propylene block copolymer, with a melt index of at least 1.0 dg/min (230xc2x0 C., 2.16 kg) and containing:
95-30 wt. % of block a1, which contains 94-100 wt. % of propylene monomer units and 0-6 wt. % of ethylene monomer units,
70-5 wt. % of block a2, which contains 30-70 wt. % of propylene monomer units and 70-30 wt. % of ethylene monomer units,
B. 0-30 wt. % of an impact modifier,
C. 0-50 wt. % of filler.
which polymer composition, if injection moulded into a beam-shaped test object at at least one value for the mass temperature chosen from 220, 240, 260 or 280xc2x0 C. and at least one value chosen for the injection time chosen from 0.2, 1.0 and 3.0 sec. in the mould for the test object having a length L of 330 mm, a width B of 30 mm and a height H of 3 mm, with a wall temperature of 40xc2x0 C., the direction of flow of the polymer composition lying in the longitudinal direction of the test object and the melt flowing into the mould consisting in vertical direction (line VD, FIG. 4) of three layers of the polymer composition, with the outer layers having a first colour and each forming 47 vol. % of the inflowing melt, and the central layer having a second colour and making up 6 vol. % of the inflowing melt, yields a test object on the surface BL of which a periodic pattern of shapes having the first colour running in longitudinal direction is absent.
The polymer composition according to the invention has a good flow, a good impact resistance, and nevertheless the polymer composition can be injection moulded at a high injection rate and with a long flow path to yield moulded parts, with no or hardly any surface defects in the form of the wave pattern.
Preferably white is chosen as the first colour, and black is chosen as the second colour to make a test object of the polymer composition of the invention.
Preferably, a value of 0.2 sec. is chosen for the injection time and the periodic pattern of shapes having the first colour is absent from the surface of the test object. Such a polymer composition can be injection moulded into moulded parts using very large flow paths and with very few or no surface defects. Even more preferably, a value of 240, 260 or 280xc2x0 C. is chosen for the mass temperature.
Preferably, the propylene block copolymer contains 80-50 wt. % of the block a1 and 50-20 wt. % of the block a2, even more preferably the propylene block copolymer contains 72-55 wt. % of the block a1 and 45-28 wt. % of the block a2.
Component B, the impact modifier, is for example a plastomer, or preferably an elastomer. Preferably, the impact modifier content is as low as possible. Preferably, its content is 0-20 parts by weight, even more preferably its content is 0-10 parts by weight. Examples of suitable elastomers are copolymers of ethylene and a second olefin with 3-10 C atoms, preferably propylene. It is also possible to use a copolymer of ethylene, propylene and one or more dienes, such as for example EPDM, as elastomer. Examples of suitable plastomers are linear low density polyethylene (lLDPE) and very low density polyethylene (vLDPE).
Preferably, the polymer composition according to the invention contains 0-15 wt. % of the elastomer and the propylene block copolymer contains 72-55 wt. % of the block a1 and 45-28 wt. % of the block a2.
As filler, use can for example be made of talc, chalk, clay, mica, clay or glass fibres and carbon fibres up to a length of 6 mm. Preferably, use is made of talc. Preferably, at least 75 wt. %, even more preferably at least 90 wt. %, of the filler particles have an aspect ratio of at least 6, even more preferably of at least 10.
Aspect ratio is understood to be the ratio between the largest and the smallest dimension of a particle.
Preferably, the melt index of the polymer composition according to the invention is at least 5, even more preferably at least 10 dg/min (230xc2x0 C., 2.16 kg).
In a preferred embodiment a polymer composition is used wherein, if the polymer composition is extruded from a reservoir having a diameter of 12 mm at at least one temperature chosen from 240, 260 and 280xc2x0 C. through a capillary with an inlet angle of 180xc2x0 having a length of 1 mm and a diameter of 1 mm with a throughput of 110 mm3/sec, the pressure drop across the capillary is at least 1.5 MPa.
Preferably, the pressure drop across the capillary is at least 1.7 MPa, more preferably at least 2.0 MPa, even more preferably at least 2.5 MPa.
Such a pressure drop can be achieved, in spite of the low viscosity of the polymer composition, for example because the Mz/Mw ratio (measured with Gel Permeation Chromatography) of the propylene block copolymer, which the polymer composition according to the invention contains, is high enough for this.
The propylene block copolymer, which the polymer composition according to the invention contains, can in itself, for example, be prepared by means of a slurry or gas phase process for the polymerization of polypropylene, with block a1 being polymerized in a first step and block a2 in a second step. For this, wellknown catalysts can be used, such as for example Ziegler catalysts on the basis of titanium or vanadium, or metallocene catalysts on the basis of zirconium. Both continuous polymerization processes and batch polymerization processes can be used. Polymerization processes for propylene block copolymers are described in themselves for example in Polymer Science 7, Polypropylene and other Polyolefins, S. van der Ven, Elsevier, Amsterdam (1990).
The high value for the Mz/Mw ratio can for example be reached by polymerizing the block a1 of the propylene block copolymer in at least two polymerization steps, with a low-molecular weight polypropylene being produced in one polymerization step and a high molecular weight polypropylene being produced in a different polymerization step, for example by choosing different temperatures for the two polymerization steps, or for example by using a low hydrogen gas content in the feed, or not using any hydrogen gas at all, in one step and using a higher hydrogen gas content in the other step. Preferably, in the first step the low hydrogen gas content or no hydrogen gas at all is used. In practice, polymerization can be effected in two steps by, for example, using two consecutive reactors with one reaction step being carried out in each of the reactors, or by carrying out the two polymerization steps in one reactor by adapting the feed after the first polymerization step. It is also possible to add a compound that reduces the catalyst""s sensitivity to hydrogen gas, for example an ester, to the reaction mixture upon completion of the first polymerization step.
Furthermore, such a polymer composition can for example be prepared by adding an ultrahigh molecular weight propylene homopolymer or random copolymer containing propylene monomers and up to 6 wt. % of ethylene monomers, with a weight average molecular weight (Mw) of at least 800,000 kg/kMol, preferably at least 1,500,000 kg/kMol, even more preferably at least 2,000,000 kg/kMol, to the propylene block copolymer in a solution, in an amount of 0.1-10 parts by weight, preferably 2-5 parts by weight, of ultrahigh molecular homopolymer or random copolymer for every 100 parts by weight of propylene block copolymer, and removing the solvent after the ultrahigh molecular homopolymer or random copolymer has been mixed with the propylene block copolymer.
The addition of ultrahigh molecular weight propylene homopolymer or random copolymer can also be performed to a part of the propylene block copolymer, to obtain a masterbatch which contains 15-40 wt % of ultrahigh molecular weight propylene polymer, whereafter the masterbatch can be mixed with additional propylene block copolymer to obtain the polymer composition of the inventon.
The moment of addition of the impact modifier and/or filler are not critical in the preparation of the polymer composition.
The invention also relates to a method in which the polymer composition according to the invention is injection moulded into moulded parts. Preferably, the polymer composition according to the invention is injection moulded into moulded parts having a thickness of 1.0-4.0 mm. The dimensionless flow path length then is preferably at least 100, more preferably 200. The dimensionless flow path length is the distance between the gate and the farthest location in the moulding to which the melt in the mould flows from the gate in question, divided by the thickness of the moulded part.
The polymer composition according to the invention is highly suitable for injection moulding into moulded parts such as, for example, a car bumper, an instrument panel, a trim and a garden table.